A broadband access may be implemented by a passive fiber optical access network, such as e.g. by a G-PON (Gigabit-capable Passive Optical Network), or an EPON (Ethernet Passive Optical Network), wherein a PON (Passive Optical Network) does not require any active electrical components for splitting the optical signals. A passive optical network typically comprises an OLT (Optical Line Termination) and one or more ONTs (Optical Network Terminals) and/or ONUs (Optical Network Units) connected by an ODN (Optical Distribution Network), which comprises optical fibers and passive optical power splitters.
The OLT is typically located at a Central Office, CO, associated with a service provider, e.g. an operator, and the CO provides an interface for delivery of services to subscribers/end-users over the PON. Each ONT/ONU terminates the PON and converts the optical signals into electrical signals for delivery of the services to terminals of the subscribers/end-users, via a suitable user interface.
The above-mentioned terms ONU and ONT both indicate a device that terminates any one of the distributed (leaf) endpoints of an ODN, implements a PON protocol, and adapts PON PDU (Protocol Data Units) to a subscriber-service interface. Further, the term CPE (Customer-Premises Equipment) may also be used for indicating an ONU or an ONT.
However, the term ONU will hereinafter be used as a generic term in this disclosure and in the appended claims, and will refer to any of an ONU, an ONT or a CPE.
An ONU of a PON is managed by the ONT Management and Control Interface (OMCI) protocol, which is a standardized PON-management protocol. The OMCI-specification ITU-T G.988 (October 2010) defines e.g. different Managed Entities, MEs, which are abstract representations of resources and services in an ONU, wherein a Managed Entity comprises a number of attributes. An OMCI message is a data packet transmitted over the ODN, wherein the data packet typically comprises one Managed Entity and control information.
The use of asynchronous packet-switched techniques has enabled an explosive growth of various telecom and datacom-services, but it has also made timing recovery more complicated. Many of the services that are available today require frequency-, phase- or time-of-day (ToD) synchronization, and the switching and multiplexing of data in packet sized quantums will result in a packet delay variation (PDV), which complicates the frequency synchronization. Further, the phase- and ToD-synchronization require a determination of the transmission delay between two nodes. However, some access technologies, e.g. G-PON, EPON and xDSL (Digital Subscriber Line), have asymmetric transmission delays, i.e. the uplink delay and the downlink delay are different, wherein a determination of the transmission delay is more complicated.
The commonly used timing protocols for packet-switched network, e.g. the IEEE 1588-2008 or the NTP (Network Time Protocol), are able to compensate for Packet Delay Variation (PDV) for frequency synchronization recovery. They may also be capable to compensate for symmetric transmission delays, i.e. a transmission delay that is essentially equal in both directions, in order to provide a phase and time-of-day synchronization. However, none of the above-mentioned timing protocols are able to compensate for asymmetric transmission delays.
Instead, access-specific methods have been introduced and standardized for overcoming the problem with the asymmetric transmission delay, e.g. in a passive optical network, when using conventional timing protocol for time-of-day synchronization. However, these methods are only capable of providing time-of-day information based on one reference clock to a remote node, such as the ONU or the CPE, i.e. in only one so-called time domain.
Hereinafter, the term time domain refers to time information based on one timing source or reference clock, and multiple time domains indicate time information based on two or more (different) timing sources or reference clocks.
In a multi-operator environment, in which one fixed network is providing Mobile Backhaul (MBH) services to client equipment served by different mobile operators, each operator may have its own timing source or reference clock. Thus, it is desirable that a passive optical network is able to transport multiple time domains, i.e. individual time-of-day information for each operator, in order to perform a time-of-day synchronization of multiple client equipment that are connected to the same remote node, but are served by different operators that may have different reference clocks.